A standard, single-junction photovoltaic cell can only be 31% efficient under AM1.5G conditions (40% under full concentration). This is due to the cell's inability to absorb photons with energy below its band gap and the fact that if a photon has energy in excess of the band gap, all additional energy is lost to the lattice as heat. FIG. 1 shows a photon being absorbed with energy equal to the band gap energy 1 and a photon being absorbed with energy greater than the band gap energy 2 and losing the additional energy as heat to the lattice 3.
Several approaches have been made to solve this problem: multi-junction cells, intermediate band cells, multiple exciton generating cells and hot carrier cells [all summarised in M. A. Green, Third Generation Photovoltaics: Advanced Solar Energy Conversion, Springer (December, 2005)]. The most successful approach has been the multi-junction cell, which has a plurality of semiconductor absorbing layers each with a different band gap. This is designed so that photons of different energies are absorbed in different layers of the cell such that the photon energy is well matched to the layer band gap and the photon energy in excess of the band gap, which is lost as heat, is minimised. However, such devices are expensive due to the difficulty of growing different semiconductor layers with optimal band gaps on top of each other.
The structure presented herein is most closely associated with the hot-carrier solar cell; an overview of the concept of the hot carrier solar cell can be found in Würfel's paper [P. Würfel et al, Progress in Photovoltaics: Research and Applications, 13[4] 277-285 (June, 2005)]. FIG. 2 shows the process by which light is absorbed in a semiconductor and creates a distribution of lattice thermalised electrons in the semiconductor's conduction band. Initially the only electrons in the conduction band are those that are thermally excited from the valence band (graph 4). After excitation with a spectrum of light with photons of different energies, electrons are excited into the conduction band of the semiconductor (graph 5), and have energies in excess of the conduction band minimum (ΔEe) determined by the difference between the photon energy (hν) and the semiconductor band gap (Eg) and the relative electron and hole masses (me,mh) (Equation 1).
                              Δ          ⁢                                          ⁢                      E            e                          =                              (                          hv              -                              E                g                                      )                    ⁢                                    (                              1                +                                                      m                    e                                                        m                    h                                                              )                                      -              1                                                          Equation        ⁢                                  ⁢        1            This non-thermal distribution of electrons thermalises by electron-electron collisions on a timescale of less than 100 fs (graph 6), at which point the electron distribution can be described by a Fermi distribution (Equation 2) with a temperature (T) well in excess of the lattice temperature.
                                          n            e                    ⁡                      (            E            )                          =                              g            ⁡                          (              E              )                                ⁢                                    (                              1                +                                  ⅇ                                                            E                      -                      μ                                        kT                                                              )                                      -              1                                                          Equation        ⁢                                  ⁢        2            This hot electron distribution then interacts with phonons and loses energy to the lattice on a timescale of 10 ps (graph 7). These lattice thermalised electrons then de-excite to the valence band of the semiconductor on a timescale of nanoseconds (graph 8) often emitting photons in the process. This process also happens analogously for the hole distribution in the valence band, with similar steps of hole-hole thermalisation and lattice thermalisation.
In a standard single band-gap solar cell, electrons are extracted from the bottom of the conduction band at location 10; i.e. after they have thermalised with the lattice and lost energy. The principle of a hot carrier solar cell is to extract them at location 9 in order to use the extra energy that would otherwise be lost by lattice thermalisation. This was first proposed in theory in 1982 [Robert T. Ross et al., “Efficiency of Hot-carrier Solar Energy Converters”, Journal of Applied Physics, 53[5] 3813-3818 (May, 1982)] and several other publications and patents have followed in a similar vein e.g.:                P. Würfel, Solar Energy Materials & Solar Cells, 46 (April, 1997)43-52: A theoretical paper extending the principle of the hot-carrier solar cell to include impact ionisation events. This reference describes energy selective “membrane” materials with “a large band gap, small bandwidth” through which contact is made. This is now exclusively realised by a resonant tunneling contact whose discrete energy levels provide an effective large band gap (due to confinement) small bandwidth contact. Extraction (tunneling) is implicitly into a metal in the structure in this paper.        G. Conibeer et al, Solar Energy Materials & Solar Cells 93 (June, 2009) 713-719: Experimental work showing resonant tunneling in Si QDs embedded in Silicon Oxide. The emitter and collector are identical (n-Si) and this is carried out with lattice thermalised carriers, but the stated aim is to use this structure for an energy selective contact.        JP04324214—PHOTOVOLTAIC FORCE DEVICE—September 2009: This reference discloses a hot carrier solar cell similar to that shown in the non-patent literature by e.g. R. T. Ross and P. Würfel, with the primary difference that JP04324214 explicitly states that the absorber layer should contain impurities (n-type or p-type) such that the majority carriers in the absorber layer will remain at substantially the same temperature upon illuminating the absorber layer with light in excess of its band gap. The aim of this device is then to selectively tunnel these low temperature majority carriers in to a contact region.        
Such devices work by extracting carriers at one particular energy (optimally the average energy) from a broad energy distribution of carriers. This extraction must happen faster than the rate of lattice thermalisation, but preferably slower than the rate of carrier thermalisation so that the energy state from which the carriers tunnel is re-populated. This isn't a strict necessity, but if the tunneling rate is faster than the carrier thermalisation then the distribution of carriers reaching the contact will be energetically wider than if the carrier thermalisation is faster than the tunneling. Hence the efficiency of such a device is higher if the carrier thermalisation rate is faster than the tunneling rate due to a lower thermalisation loss in the contacts.
In all such papers and patents the method of hot carrier extraction is via energy selective tunneling. With further reference to FIG. 3, this is brought about by placing quantum well layers 12 and 14 adjacent to the absorber layer 13 such that tunneling of carriers occurs through the discrete energy states of the wells. The tunneling explicitly or implicitly occurs directly into electrical contacts 11 and 15. In the exemplary device of FIG. 3, carriers are photo-generated in the absorber layer 13 to form hot carrier distributions 16 and 17, from which carriers then selectively tunnel into the contacts 11 and 15 at energies Eh and Ee respectively
No device has yet been constructed whereby hot carrier selective tunneling has taken place from a semiconductor into a contact with a higher Fermi level than the semiconductor. However, tentative evidence for hot carrier selective tunneling between two identical semiconductor regions has been presented by S. Yagi [S. Yagi and Y. Okada, Fabrication of resonant tunneling structures for selective energy contact of hot carrier solar cell based on III-V semiconductors, Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, 1213-1216 (June, 2010)]. The structure used by S. Yagi is similar to FIG. 4, and so is not suitable for use in a hot carrier photovoltaic cell, since lattice thermalisation of the carriers would happen in the absorber layer 21 after tunneling. However, it does show tentative proof of principle that selective tunneling of hot carriers can occur between two regions faster than lattice thermalisation.